Voltage reference trim

[Freq Band]

Even though I am using two 10-turn pots, I am not getting the fine resolution I desire with the trim network shown here.
I would like the "fine" trim to be able to have a 1mV range over the pot's entire span.
(which I believe will give me 100uV trim per turn, on a 10-turn pot.)

Any suggestions?

=FB=

 

[Freq Band]

I put a 1meg resistor between "trim pin" and wiper of the first pot.
This helps greatly.
How does this change the circuit ?

=FB=

 

[PRR]

I'm going to ignore your R1 because it is a fine trim on the nominal "5.000V".

You start with 5V and you want 100uV/turn??? 5V/100uV= a 50,000-turn pot.

One 10-turn won't do it.

Two 10-turns can fake a 10*10= 100-turn pot.

I think you need five 10-turn pots.

Or a re-examination of what you really need: a micrometer or a surveyor's chain.

You want some variation on this.

But basically your R2 is 5V-0V, so 0.5V/turn. Do you really need 5V-0V here?

Given 0.1% (1/100th turn) resolution in R2, the minimum jump is 5mV.

The pot I added will multiply by 1.00 to 0.9901. At 5V in that is 5.00V-4.95V, a 50mV span, 5mV/turn.

Change 1,000K (1meg) to 10Meg, you get 5mV span, 0.5mV/turn, 5 microvolt nominal setability.

 

[PRR]

> I put a 1meg resistor between

I was going to say.... that direct-connect is NOT how the sheet shows it; and you may well have burned or drifted the precision internal divider with careless pot-turning.

> How does this change the circuit ?

A smart geek like you should learn to solve any Voltage-Divider problem on a cocktail napkin.

Your statement of problem is incomplete without saying what range you want XX resolution over.

 

[Freq Band]

You're right, I should do the divider calc.....but not tonight, I'll make an error ('zzzzzzzz)

The 1meg to wiper....I was trying to steal the idea in this model......

Yes, I want the full range of trim, 0-5V. (Circuit here above is idea for "fine" portion only).

=FB=